Poly profile engineering to modulate spacer induced stress for device enhancement

ABSTRACT

The present invention provides a method of inducing stress in a semiconductor device substrate by applying an ion implantation to a gate region before a source/drain annealing process. The source/drain region may then be annealed along with the gate which will cause the gate to expand in certain areas due to said ion implantation. As a result, stress caused by said expansion of the gate is transferred to the channel region in the semiconductor substrate.

BACKGROUND OF THE INVENTION

The present invention relates to a method of fabricating semiconductorstructures and, more particularly, to a method of enhancingsemiconductor device performance by modulating spacer induced stress.

Minimum feature sizes of CMOS semiconductor devices are continuouslybeing reduced. To overcome limitations of the scaling of CMOS devicesvarious techniques have been developed to enhance carrier mobility. Onesuch technique is to induce stress in the channel region to enhancecarrier mobility. Various ways to induce stress in the channel regionhave been developed including: stress engineering through local straintechniques such as dual stress liners (DSL), and stress proximitytechnique (SPT); process induced strain methods such as stressmemorization technique (SMT); and recessed SiGe source/drain for PMOS.

The SPT and DSL techniques have been demonstrated to improve both NFETand PFET drive currents. The reduced proximity of stress liner due tothe removal of the spacer before the DSL process maximizes the straintransfer from nitride liner to the channel.

In the SMT process, the stress effect from the highly tensile nitridecap layer is enhanced and memorized by well-controlled poly gateamorphization and re-crystallization steps.

U.S. Pat. No. 6,975,006 to Huang et al. discloses the use of recessedfirst and second recessed spacers situated on top of first and secondpoly gate sidewall liners respectively. The height of the originallyformed spacers is reduced for the recessed spacers. Also, the width ofthe horizontal part of the sidewall liner is shorter than the width ofthe spacer. It is asserted that the reduced spacer height can reduce thedevice channel stress.

U.S. Pat. No. 7,132,704 to Grudowski discloses the use of a tensilenitride spacer to enhance the NFET device performance. The stress in thetensile nitride spacers film can be adjusted with selectively ornon-selective implantation of xenon or germanium either before or afteretching the spacer film.

One disadvantage with prior techniques for modulating stress induced inthe channel region is that they are relatively complex. Anotherdisadvantage is that there is currently no simple technique, to theinventors' knowledge, to separately modulate the stress on variousdevices of different threshold voltages, such as for low, regular andhigh threshold voltages (LVT, RVT and HVT respectively). Anotherdisadvantage with prior stress modulating techniques is that they cannotalways be used in combination with other such techniques.

As can be seen, there is a need for an improved method of modulatingspacer induced stress for device enhancement. Further, there is a needfor such a technique which is not complex and with can allow theseparate modulation of stress on various devices of different thresholdvoltages. Also, there is a need for a stress modulation technique thatcan be used in combination with other stress modulation techniques.

SUMMARY OF THE INVENTION

In one aspect of the present invention, a method of processing asemiconductor device comprises forming a polysilicon film on asemiconductor substrate; forming a photo resist layer over a firstpredetermined region of the semiconductor substrate; forming a mask overthe first predetermined region of the semiconductor substrate; ionimplanting the polysilicon film over a second predetermined region ofthe semiconductor substrate, the second predetermined region beingdifferent from, and adjacent to, the first predetermined region; andannealing a gate formed on the implanted polysilicon film.

In another aspect of the present invention, a method of processing asemiconductor device comprises: forming a polysilicon film on asemiconductor substrate; forming a photo resist layer over a firstpredetermined region of the semiconductor substrate; forming a mask overthe first predetermined region of the semiconductor substrate; ionimplanting the polysilicon film over a second predetermined region ofthe semiconductor substrate, the second predetermined region beingdifferent from, and adjacent to, the first predetermined region; andannealing a gate formed on the implanted polysilicon film.

In accordance with a further aspect of the present invention, atransistor comprises: a semiconductor substrate; a source in thesemiconductor substrate; a drain in the semiconductor substrate; and agate on the semiconductor substrate, the gate having bulging sidewalls.

These and other features, aspects and advantages of the presentinvention will become better understood with reference to the followingdrawings, description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-sectional representation of a transistor fabricatedaccording to a first embodiment of the stress modulating method of thepresent invention;

FIG. 2 shows a cross-sectional representation of a transistor fabricatedaccording to a second embodiment of the stress modulating method of thepresent invention;

FIG. 3 shows a cross-sectional representation of a transistor fabricatedaccording to a third embodiment of the stress modulating method of thepresent invention;

FIG. 4 is a micrograph of a cross-section of a PFET transistor gateshowing the effect of the stress modulation method of the presentinvention;

FIG. 5 is a micrograph of a cross-section of an NFET transistor gateshowing the effect of the stress modulation method of the presentinvention; and

FIG. 6 shows a flow chart of a method of modulating the stress in atransistor in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description is of the best currently contemplatedmodes of carrying out the invention. The description is not to be takenin a limiting sense, but is made merely for the purpose of illustratingthe general principles of the invention, since the scope of theinvention is best defined by the appended claims.

Broadly, the present invention may be advantageously used insemiconductor fabrication processes for producing devices having highcarrier mobility. The present invention may also be advantageously usedin CMOS device fabrication where it is desired to induce and modulatethe stress in the channel region.

Embodiments of the present invention may provide a method to improvecarrier mobility by modulating the stress in the channel region of atransistor. To accomplish this, the present invention may employdifferent Germanium (Ge) implant conditions to the poly gate, followedby subsequent source/drain anneal. As a result of this process, theshape of the poly gate may be changed, which induces stress on theadjacent spacer thereby inducing stress on the channel. Furthermore,using this method the spacer induced stress in the channel may becontrolled for various devices on a wafer independently, for example forLVT, RVT and HVT devices. Another advantage is that the process can berelatively simple, as compared to other techniques for inducing channelstress. A further advantage is that the present invention can beincorporated and used together with various other stress enhancementtechniques, such as SMT.

Prior art semiconductor fabrication methods for inducing channel stresswere relatively complex and did not employ different Ge implantconditions to the poly gate, followed by subsequent source/drainannealing.

FIGS. 1 a and 1 b show cross-sectional views of partially fabricatedCMOS transistor structures 10, 12 respectively in accordance with anembodiment of the invention. It will be appreciated that transistorstructures 10, 12 may each be part of a semiconductor wafer (not shown)containing a large number of identical transistor structures, as well asother semiconductor structures.

Referring now to FIG. 1 a, at this stage the transistor structure 10 maycomprise a semiconductor substrate 14 having an N-well 16 and a P-well18 separated by dielectric isolation structures 20. The region of thesemiconductor substrate 14 containing the N-well 16 is the target, orPFET region 17. The region of the semiconductor substrate 14 containingthe P-well 18 is the non-target or NFET region 19. When thesemiconductor structure 10 is complete, as shown for example in FIG. 3a, it can be a PFET device.

A gate oxide layer 22, which may comprise silicon dioxide, may be formedoverlying the semiconductor substrate 14. The gate oxide layer 22 may beformed by thermal oxidation or by chemical vapor deposition (CVD). Apolysilicon layer 24 may be formed over the gate oxide layer 22, whichwill eventually be used to form the gate of the transistor structure 10.The polysilicon layer 24 may be deposited using, for example, alow-pressure chemical vapor deposition (LPCVD) process.

A photo resist layer 26 may be deposited over the NFET region onto thepolysilicon layer 24 using conventional techniques well known in theart, such as spin-coating the photo resist, followed by the lithographicprocess of exposing the resist using a photo-reticle (mask) and thendeveloping away the exposed resist. An implant mask 28 may be placedover the photo resist layer 26. The mask 28 may be made of a materialsuitable for blocking the ions subsequently implanted into thepolysilicon layer 24 as described below.

Once the transistor structure 10 has been formed with theabove-described features using conventional techniques, a polyamorphorization ion implantation step may be performed in accordancewith one embodiment of the invention. In particular, in this embodimentof the invention, Ge⁺ ions 30 may be used for the ion implantation step.Other ions, such as Xe⁺, may also be used.

The Ge⁺ implantation on the polysilicon layer 24 may be performed usingvarying doses and energies depending on the desired results. Forexample, different doses and energies may be used for the ionimplantation depending on whether the transistors being fabricated areHVT, RVT or LVT devices. In one embodiment, these different devices maybe fabricated on the same wafer by using separate masking layers for thedifferent types of devices. For example, when performing the ionimplantation on the polysilicon layer 24 for an HVT device, the mask 28may be configured to block the polysilicon layers for the non-HVTdevices. In this way, the different Ge dose and energy can be applied tothe polysilicon layers in the HVT, RVT and LVT devices.

The transistor structure 10 may then be processed in accordance withconventional CMOS fabrication techniques to complete the device, asdescribed in more detail below. However, because of the ion implantationthat was performed on the polysilicon layer 24 in accordance with thisembodiment of the invention, when the reoxidation and source/drainanneal steps are performed, the shape of the gate formed from thepolysilicon layer 24 may be altered, in particular, the gate may haveexpanded regions. It is this change in the shape of the gate that mayinduce stress in the channel region as described in more detail below.

FIG. 1 b, includes the same elements as in FIG. 1 a except that the mask28 may be positioned over the PFET region 17 and the ion implantation 30is performed on the polysilicon layer 24 over the NFET region where thegate will be formed. When the semiconductor structure 12 is complete itcan be an NFET device.

FIGS. 2 a and 2 b show cross-sectional views of partially fabricatedCMOS transistor structures 32, 34 respectively, in accordance withanother embodiment of the invention. The CMOS transistor structures 32,34 differ from those shown in FIGS. 1 a and 1 b in that they are shownat a later stage of processing. Also, the poly amorphorization ionimplantation of the present embodiment may be performed at a later stagethan in the embodiment shown in FIGS. 1 a and 1 b.

Referring specifically to FIG. 2 a, at this stage, the transistorstructure 32 may comprise a semiconductor substrate 33 having an N-well36 and a P-well 38 separated by dielectric isolation structures 40. Theregion of the semiconductor substrate 33 containing the N-well 36 is thePFET region 42. The region of the semiconductor substrate 33 containingthe P-well 38 is the NFET region 44.

A gate oxide layer 46, which may comprise silicon dioxide, may be formedoverlying the semiconductor substrate 33. The gate oxide layer 46 may beformed by thermal oxidation or by chemical vapor deposition (CVD). Apolysilicon gate 48 may be formed by patterning a polysilicon layer thatwas formed over the gate oxide layer 46 using conventional techniques.

A photo resist layer 50 may be deposited over the NFET region 44 onto anarea of the semiconductor substrate 33. The photo resist layer 50 may beformed over a polysilicon gate 52 formed over a gate oxide layer 54using conventional techniques well known in the art, such asspin-coating the photo resist, followed by the lithographic process ofexposing the resist using a photo-reticle (mask) and then developingaway the exposed resist. An implant mask 56 may be placed over the photoresist layer 50. The mask 56 may be made of a material suitable forblocking the ions subsequently implanted into the polysilicon gate 48,as described below.

Once the transistor structure 32 has been formed with theabove-described features using conventional techniques, a polyamorphorization ion implantation step may be performed in accordancewith this embodiment of the invention. In particular, Ge⁺ ions 58 may beused for the ion implantation step. Other ions may also be used. Asdiscussed above in connection with the embodiments shown in FIGS. 1 aand 1 b, when subsequent processing takes place, the shape of thepolysilicon gate 52 may change, for example by expanding laterally incertain regions resulting in stress in the channel region as describedin more detail below. Also, in a similar manner as described along withthe embodiments shown in FIGS. 1 a and 1 b, the Ge implantation may beperformed using varying doses and energies to cause expansion of thepolysilicon gate 52 and thereby modulate the stress in the channel.

FIG. 2 b, includes the same elements as in FIG. 2 a except that the mask56 may be positioned over the PFET region 42 and the ion implantation 58may be performed on the polysilicon gate 48 over the NFET region. Whenthe semiconductor structure 34 is complete it can be an NFET device.

FIGS. 3 a and 3 b show cross-sectional views of partially fabricatedCMOS transistor structures 58 and 60 respectively in accordance withanother embodiment of the invention. The CMOS transistor structures 58,60 differ from those shown in FIGS. 2 a and 2 b in that they are shownat a later stage of processing and the poly amorphorization ionimplantation of the present invention may be performed at a later stagethan in the embodiment shown in FIGS. 2 a and 2 b.

Referring specifically to FIG. 3 a, at this stage, the transistorstructure 58 may comprise a semiconductor substrate 62 having an N-well64 and a P-well 66 separated by dielectric isolation structures 68. Theregion of the semiconductor substrate 58 containing the N-well 64 is thePFET region 70. The region of the semiconductor substrate 58 containingthe P-well 66 is the NFET region 72.

A gate oxide layer 74, which may comprise silicon dioxide, may be formedoverlying the semiconductor substrate 58. Two polysilicon gates 76 maybe formed by patterning a polysilicon layer that was formed over thegate oxide layer 74 using conventional techniques. The two polysilicongates 76 may be formed over the PFET region 70 and over the NFET region72.

Source 78 and drain 80 regions may have been formed in the PFET and NFETregions 70, 72 using conventional techniques. Conventional sidewallspacers 82, 84 along with conventional sidewall liners 86, 88 may beformed on each side of both polysilicon gates 76.

A photo resist layer 90 may be deposited over the NFET region 72 onto anarea of the semiconductor substrate 33 covering the polysilicon gate 76disposed over the NFET region 72 using conventional techniques wellknown in the art. A mask 92 may be placed over the photo resist layer50. The mask 92 may be made of a material suitable for blocking the ionssubsequently implanted into the polysilicon gate 48 as described below.

Once the transistor structure 58 has been formed with theabove-described features using conventional techniques, a polyamorphorization ion implantation step may be performed in accordancewith this embodiment of the invention. In particular, Ge⁺ ions 94 may beused for the ion implantation step. Other ions may also be used. Asdiscussed above with regards to the embodiments shown in FIGS. 2 a and 2b, when subsequent annealing processing takes place, the shape of thepolysilicon gate 76 may change, for example by expanding laterally incertain regions resulting in stress in the channel region as describedin more detail below. Also, in a similar manner as described along withthe embodiments shown in FIGS. 2 a and 2 b, the Ge implantation may beperformed using varying doses and energies followed by subsequentannealing process(es) to modify the shape of the polysilicon gate 76 andthereby modulate the stress in the channel.

FIG. 3 b, includes the same elements as in FIG. 3 a except that the mask92 may be positioned over the PFET region 70 and the ion implantation 94may be performed on the polysilicon gate 76 over the NFET region 72.When the semiconductor structure 60 is complete it can be an NFET device

FIG. 4 shows a tunneling electron microscope (TEM) image 96 of across-section of a PFET polysilicon gate 98 showing the effect of thestress modulation technique of the present invention. In particular,when processing the transistor structures according to the presentinvention as shown in FIGS. 1 a, 2 a, and 3 a, the resulting PFET devicemay have a polysilicon gate 98 disposed on a semiconductor substrate 99with its shape modified as shown in FIG. 4. Contour 100 shows that theside edges of the polysilicon gate 98 with the Ge ion implantationperformed on it may have expanded in an upper portion 101 aftersubsequent source/drain anneal in accordance with the present invention.Contour 102 shows that the side edges of the polysilicon gate 98 withoutthe Ge ion implantation performed on it remains flat in both the upperportion 101 and a lower portion 103. The bulging of the sides caused bythe Ge ion implantation and subsequent anneal, may cause increasedresidual compressive stress in the transistor channel (not shown) withinthe semiconductor substrate 99. The bulge in the polysilicon gate is dueto localized expansion of the polysilicon gate during there-crystallization process that occurs during annealing as the implantedGe ions that are larger than the Si atoms attempts to fit into the Silattice. This bulging is primarily near the top of the polysilicon gate98 because the peak of the implanted Ge ions is located near the top ofthe PFET polysilicon gate. Diagrammatic lines illustrating the forcescausing this stress are also illustrated in FIG. 4. In particular, lines104 show the outward lateral forces on the adjacent spacer (not shown),lines 106 show the downward vertical forces, and lines 108 show theinward lateral forces exerted in the channel within the semiconductorsubstrate 99.

FIG. 5 shows a tunneling electron microscope (TEM) image 110 of across-section of an NFET polysilicon gate 112 showing the effect of thestress modulation technique of the present invention. In particular,when processing the transistor structures according to the presentinvention as shown in FIGS. 1 b, 2 b, and 3 b, the resulting NFET devicemay have a polysilicon gate 112 disposed on a semiconductor substrate114 with its shape modified as shown in FIG. 5. Contour 116 shows thatthe side edges of the polysilicon gate 112 with the Ge ion implantationperformed on it may have expanded in the lower portion 103 aftersubsequent source/drain anneal in accordance with the present invention.Contour 118 shows that the side edges of the polysilicon gate 112without the Ge ion implantation performed on it remain flat in upper andlower portions 101, 103. The bulging of the sides caused by the Ge ionimplantation and subsequent anneal, may cause increased residualcompressive stress in the transistor channel (not shown) within thesemiconductor substrate 99. In particular, this bulging may be primarilynear the bottom of the polysilicon gate 112 because the peak of theimplanted Ge ions is located near the bottom of the PFET polysilicongate. Diagrammatic lines illustrating the forces causing this stress arealso illustrated in FIG. 5. Lines 120 show the outward lateral forces onthe adjacent spacer (not shown), lines 122 show the upward verticalforces, and lines 124 show the outward lateral forces exerted in thechannel within the semiconductor substrate 99.

FIG. 6 shows a flow chart of a process 126 of modulating spacer inducedstress in accordance with one embodiment of the invention. Process 126begins with the conventional processing of a semiconductor substrate toa desired stage in step 128. For example, the processing may be up tothe stage as shown in FIGS. 1 a, 1 b, or FIGS. 2 a, 2 b or FIGS. 3 a, 3b. A photo resist layer may be applied over the transistor substrate inareas except where the target gate to be ion implanted is located asshown in step 130. For example, this may comprise photo resist 26, 50 or90 discussed above. In step 132 a mask may be applied over the same areaas the photo resist. This may comprise, for example, mask 28, 56 or 92discussed above. Ion implantation, step 134, may then be performed tothe target gate area, such as ion implantation 30, 58 or 94 discussedabove. In step 136 the processing of the transistor device may becompleted in a conventional manner.

Thus, it may be seen that the present invention may provide a method ofmodulating spacer induced stress by ion implanting the polysilicon gate.The resultant change in the shape of the gate may induce stress on thespacers, which in turn may increase stress in the channel to enhancecarrier mobility. The technique is simple and can be separatelyimplanted to HVT, RVT and LVT devices to suitably and independentlymodify the polysilicon gate profile to obtain the desired deviceperformance and enhancement.

It should be understood, of course, that the foregoing relates toexemplary embodiments of the invention and that modifications may bemade without departing from the spirit and scope of the invention as setforth in the following claims.

1. A method of inducing stress in a semiconductor device substratecomprising: applying an ion implantation to a target gate region beforea source/drain annealing process; source/drain annealing said targetgate region wherein a gate formed in said target gate region is expandeddue to said ion implantation; and completing the fabrication of asemiconductor device on said semiconductor substrate, whereby stresscaused by said expansion of said gate is transferred to saidsemiconductor substrate.
 2. The method of claim 1 wherein said applyingcomprises applying ion implantation to a polysilicon film.
 3. The methodof claim 1 wherein said applying comprises applying ion implantation toa polysilicon gate.
 4. The method of claim 3 wherein said applyingcomprises applying ion implantation to a polysilicon gate havingsidewall spacers.
 5. The method of claim 1 wherein said applyingcomprises applying a germanium ion implantation.
 6. The method of claim1 wherein said annealing comprises increasing the width of said gate. 7.The method of claim 1 wherein said semiconductor device is an NFETdevice and said annealing comprises increasing the width of the upperportion of said gate.
 8. The method of claim 7 further comprisingapplying downward forces on said semiconductor device substrate adjacentto said gate.
 9. The method of claim 1 wherein said semiconductor deviceis a PFET device and said annealing comprises increasing the width ofthe lower portion of said gate.
 10. The method of claim 9 furthercomprising applying upward forces on said semiconductor device substrateadjacent to said gate.
 11. A method of processing a semiconductor devicecomprising: forming a polysilicon film on a semiconductor substrateincluding target and non-target regions; forming a photo resist layerover said non-target region of said semiconductor substrate; forming amask over said non-target region of said semiconductor substrate; ionimplanting said polysilicon film over said target region of saidsemiconductor substrate, said second predetermined region beingdifferent from, and adjacent to, said first predetermined region; andannealing a gate formed on said implanted polysilicon film.
 12. Themethod of claim 11 further comprising expanding the dimensions of saidgate during said annealing.
 13. The method of claim 12 furthercomprising forming sidewall spacers adjacent to said gate.
 14. Themethod of claim 13 further comprising transferring forces caused by saidexpanding of the dimensions of said gate to said sidewall spacers. 15.The method of claim 14 further comprising transferring said forcesthrough said sidewall spacers to said semiconductor substrate below saidsidewall spacers.
 16. The method of claim 11 further comprising:repeating said steps of forming a photo resist layer, forming a mask andion implanting on different regions of said semiconductor substrate andunder different ion implantation conditions, said conditions includingat least one of: ion implantation dosages and ion implantation energies.17. The method of claim 16 wherein said different regions of saidsemiconductor substrate having ion implanting under different conditionsinclude regions of semiconductor devices including at least two of thefollowing: low voltage threshold devices, medium voltage thresholddevices and high voltage threshold devices.
 18. A transistor comprising:a semiconductor substrate; a source in said semiconductor substrate; adrain in said semiconductor substrate; and a gate on said semiconductorsubstrate, said gate having bulging sidewalls.
 19. The transistor ofclaim 18 wherein said bulging sidewalls are disposed near the top ofsaid gate.
 20. The transistor of claim 20 wherein said bulging sidewallsare disposed near the bottom of said gate.